Entrevista Aplicada al proyecto de Investigación

3.5.1. Methodology

Submission two dealt with the comparison of AA7020 and V3C by means of Vickers hardness testing. Tests were conducted to establish a suitable aging treatment after forming to meet the 300 MPa yield strength target set for the project. Testing was conducted to establish if a short aging treatment at higher temperatures would be suitable but the alloys did not respond well to this. A two stage treatment based around previous work elsewhere [72] and talks with industrial partners [73] of 90°C and 130°C for 8 and

18 hours was established as the optimum treatment for the alloys to achieve peak strength and hardness.

Hardness testing in submission three looked at the influence of reducing solution treatment time, (which mimics the heating cycle during forming) on material strength after aging to establish the shortest cycle time that could be used to still achieve peak strength. The influence of nickel additions within the V3C on the quench rate sensitivity of the alloys was investigated by using both water and air quenches after solution treatment. Tensile testing was conducted on AA7020 to establish a conversion value from hardness to yield strength but this was not possible with V3C due to a lack of experimental material.

Vickers hardness and tensile testing was then conducted on AA7020 and V3CN to compare the two materials and to establish a conversion for the V3CN between the two values. The hardness values were used to compare the V3C to the V3CN to look at the difference in strength after scale up of production. Vickers hardness was then used to establish the influence of a customer’s paint bake cycle on material strength, which was used to modify the aging treatment to ensure a final strength after paint bake of 300 MPa yield.

Samples of alloys were subjected to various solutionization temperatures to mimic the heating achieved during a forming cycle for various times, followed by various aging cycles and finally by a simulated paint bake cycle detailed in fig.36. The details of sample treatments are detailed with results in the next section of this report.

Fig.36. Simulated customer paint bake heat cycle temperatures in°C labelled.

After these heat treatments samples were prepared for hardness testing by sectioning, mounting in resin and then polished. Vickers hardness testing was conducted using a 10 kg mass with a ten second dwell time using a Buehler automatic hardness tester an average of nine indents of each sample was taken.

Samples for tensile testing were machined into tensile dogbones according to BSI standard ISO 6892-1 [74] the dimensions of which are shown in fig.37 all samples had a nominal thickness of 1.6 mm.

Fig.37. Tensile dogbone sample geometry.

Samples were then loaded into an INSTRON 6652 load frame equipped with a 100 kN load cell and 100 kN tensile grips, Instron Bluehill V.2.4 software was used to control the test profile. The frame was zeroed and the relevant test profile was run with a cross head speed of 13.5 mm/min. This was an established test profile used for yield strength testing and tests were repeated three times.

180 mm

120 mm

80 mm

3.5.2. Results

Fig.38. Hardness vs. solution treatment time at 500°C for AA7020 and V3C.

Initial tests to establish the shortest solutionization time to achieve peak hardness are shown in fig.38 with no difference observed between 30 minutes and 5 minutes solution treatment. Following this testing solution treatment times were further decreased to look at the influence of this decreased time, and to assess the feasibility of further reduced cycle times for an industrial process. The results of this testing are shown in fig.39.

Fig.39. Hardness vs. solution treatment time at 500°C for AA7020, V3C and V3CN.

Fig.40. Yield strength vs five minute solution treatment temperature of AA7020 and V3CN.

The results of room temperature tensile testing to establish the material yield strength and the effect of solutionization temperature are shown in fig.40. The results of this testing

were compared with hardness testing under the same conditions to give a conversion value for later testing both at WMG and Superform.

A simulated customer paint bake cycle as detailed in fig.36 was applied to both alloys after the two stage aging treatment. The length of aging treatment was then varied as detailed in table 13 (customer age refers to the paint bake cycle) to look at the influence on final strength after the paint bake cycle, the results of which are shown in fig.41

Table.13. Heat treatment cycles after simulated paint bake before tensile testing.

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Fig.41. AA7020 and V3CN yield strength after paint bake and aging as detailed in table 13.

Sample Solutionized Pre-Age Pre-Bake Artifical Age Customer Age A 15 min @ 500 °C (503°C) 72 hrs @ RT 24 hours @ 40 °C 8 hours @ 90 °C + 18 hours @ 130 °C None

B 15 min @ 500 °C (503°C) 72 hrs @ RT 24 hours @ 40 °C 8 hours @ 90 °C + 18 hours @ 130 °C As detailed

C 15 min @ 500 °C (503°C) 72 hrs @ RT 24 hours @ 40 °C 8 hours @ 90 °C + 16 hours @ 130 °C As detailed

D 15 min @ 500 °C (503°C) 72 hrs @ RT 24 hours @ 40 °C 8 hours @ 90 °C + 14 hours @ 130 °C As detailed

E 15 min @ 500 °C (503°C) 72 hrs @ RT 24 hours @ 40 °C 8 hours @ 90 °C + 12 hours @ 130 °C As detailed

3.5.3. Discussion

Initial testing identified the two stage 90°C for 8 hours and 130°C for 18 hours as the optimum artificial aging to achieve peak hardness in both alloys. Testing for varied amounts of solutionization time proved that both alloys could achieve peak hardness within the five minute target cycle time but if the solution treatment time was reduced further there was a detrimental effect on the final hardness values.

A peak yield strength of 305 MPa for the AA7020 alloy was observed after a 5 minute solution treatment at 500°C followed by the two stage aging treatment, whereas for the V3CN the peak yield strength of 346 MPa was achieved after 5 minutes at 480°C followed by the two stage treatment. The V3CN after solutionization at 500°C achieved a strength of 342 MPa yield, the alloy was observed to be stronger/harder than the AA7020 alloy under all conditions tested. Following testing conversion rates from hardness to yield strength were found to be 2.87:1 for AA7020 and 2.76: for V3CN.

Following simulated paint bake cycle tests the AA7020 showed slight improvement in strength with the shortest aging treatment of 8 hours at 90°C and 10 hours at 130°C achieving 317 MPa, suggesting that with the standard treatment and consequent paint bake the material is being slightly overaged leading to a slight reduction in strength. The alloy across all tests met the required 300 MPa yield target with the lowest strength being 303 MPa with the 8 hours at 90°C and 12 hours at 130°C aging treatment.

The V3CN saw a peak yield strength of 342 MPa following the standard two stage aging treatment with no paint bake cycle, all samples with the paint bake cycle applied showed decreased strength. This suggests over aging of the alloy with the paint bake cycle, this was substantiated by the recovery of yield strength with reduction of aging time, the alloy

achieving a yield strength of 339 MPa with the shortest 8 hours at 90°C and 10 hours at 130°C aging treatment and subsequent paint bake.

3.5.4. Conclusion

Both AA7020 and V3CN alloys can achieve the required 300 MPa yield strength target after a 5 minute solutionization treatment to simulate the heating cycle during forming. This means that both alloys would meet the strength and cycle time criteria for the project, their commercial viability then depending upon the alloy formability.

A 1.6wt% nickel addition in V3C alloy was shown to significantly improve the strength and hardness of the alloy, with testing in all conditions offering greater strength than AA7020. Unlike the formability of the material the influence of scaling up production in the V3CN did not detrimentally effect the alloy strength, instead further increasing the strength.

The addition of nickel caused a slight increase in quench rate sensitivity from 3% to 5% in comparison with the AA7020 alloy. This increase would not be of consequence commercially as the V3CN alloy is well in excess of the 300 MPa yield strength requirement even when air quenched.

Both alloys achieve the required final 300 MPa yield strength target after a simulated customer paint bake heat treatment when a modified two stage aging treatment with a decreased duration of the second stage is employed. This reduction in aging duration helps to prevent over aging of the alloy during customer processing of the alloys.